Scientific Method —

It’s (just barely, sort of) alive!

Below the ocean floor, microbes are surviving on 100,000 oxygen molecules a day.

For something we are all familiar with, life tends to be a remarkably difficult thing to define. Most definitions, however, include the ability to reproduce. Which means that a microbial community that has been discovered on the floor of the Pacific Ocean stretches the definition of what it means to be alive.

Bacteria are detected in the sediments down to the level of at least 20m, and are probably present (if rare) below that. But based on oxygen consumption, the cells are operating at a metabolism that appears to be right at the minimum energy flux needed to simply keep their cellular components operational. With no energy to spare, it's possible that these cells are not even able to reproduce.

The discovery of thriving ecosystems at deep-ocean hydrothermal vents helped revolutionize how we view life. Unlike familiar ecosystems on the surface, these organisms weren't ultimately dependent upon sunlight to power the base of the food chain, instead relying on chemical energy provided by the Earth's internal heat. Since then, other communities have been found that rely on unusual sources of energy. Deep in a mine, organisms appear to rely on radioactive decay to provide a source of hydrogen. Under an Antarctic ice sheet, another community grabs its energy by oxidizing iron exposed by the glaciers.

These organisms have provided us with insights about the broad range of energy sources that can be used to support life. But they've also helped tell us about just how little energy life needs to squeak by. The bacteria found in the South African mine, for example, get so little energy that they're probably able to divide only once every few hundred years. In contrast, a well-fed E. coli can divide every 20 minutes.

The newly described bacterial community may make those look positively energetic.

They were spotted by a research cruise that was taking sediment cores across the equatorial Pacific before turning north and sampling up past Hawaii. This path took it into the North Pacific Gyre, an area that has been made famous because circulating currents tend to trap a lot of our trash there. But biologically speaking, it was also an area of low biological activity. The surface waters are relatively nutrient-starved, and have a primary productivity that is nearly an order of magnitude lower than the open ocean nearby.

On the ocean floor, this seems to have produced a completely different ecosystem than many other areas of the globe. Generally, the sediment that falls to the ocean floor is rich in organic compounds created by the life above. These get rapidly digested by the microbes above, which burn through the sediments oxygen at a rapid clip while doing so. As a result, just a short distance from the surface, the oxygen gets very sparse, and anaerobic microbes take over.

The sediment in the gyre is nothing like that. It takes roughly a thousand years for just a millimeter to accumulate, and bacterial activity is so low that there's oxygen down to at least 30 meters—at which point, the sediment was probably deposited about 86 million years ago. If big numbers are a problem, think of it this way: that's back when dinosaurs dominated terrestrial ecosystems.

The authors searched their sediments for bacteria, and were able to detect them down to at least 20m, at which point there were only about 1,000 cells in every cubic centimeter of muck. There may be more below that, but they were below the authors' ability to detect them. Based on the density of the cells present and the rate at which oxygen decreased with depth, the authors were able to calculate how much oxygen was being used to keep these cells alive.

The results were pretty eye-popping. Thirty meters below the ocean floor, an entire liter of sediment would only burn through a nanomole of oxygen—in an entire year. At that rate, a single cell would be using just 100,000 oxygen molecules a day to stay alive. That's below the rate described for the slowest-growing cultures we've managed to maintain in a lab and probably closer to the oxygen use of bacteria in a quiescent, stationary phase of growth. "These microbial communities may be living at the minimum energy flux needed for prokaryotic cells to subsist and that the total available energy flux ultimately controls the microbial community size in the deep biosphere," the authors conclude.

In other words, they've only been receiving enough energy to do maintenance, and probably not enough to actually divide. Which stretches most definitions we have of what it means to be alive.

The biggest question lingering over the story is what's actually going on once oxygen use reaches its steady state level in these sediments. Given the difficulty of detecting cells, the authors haven't clearly demonstrated that a non-biological process doesn't take over below a certain depth. Gently adding some nutrients and a bit more oxygen might allow something to grow out of the sample, providing a clearer indication that there really is something alive down there.

And, if something does grow out, then we could actually subject it to DNA sequencing and figure out what exactly can live under such extraordinarily limiting conditions.

For comparison, say that in one breath we breath about a liter of air. At 25C a mole of air would occupy 24.5 liters. Say air is 1/5th oxygen, that we breath about 12 times/minute# and that we use about 1/4 of the oxygen in the air##, we get: (1/24.5) * (1/5) * 1/4 * 12 * 60 * 24 * 365 * 10^^9 ~= 13 trillion nano moles of oxygen used by a human in a year.

Or, humans use about 13,000 billion billion times as much oxygen as these mud bugs in a given unit of time.